1. Field of the Invention
This invention relates to compact disc recordings, and specifically to a means for enhancing the reproduction of sounds from compact discs.
2. Description of the Prior Art
Compact discs or CDs are recordings used in compact disc players having a laser with associated electronic circuitry to read information, such as recorded music, stored on the compact discs. Compact discs generally are created by digitally sampling the information or music source to be recorded, and recording the sampled signal as a series of pits and lands of variable length formed on a polycarbonate plastic substrate coated with an aluminum reflective coating and protected by a layer of clear lacquer. A land is the surface of the substrate between the pits. Usually, the digitally sampled recorded information is represented by transitions between the pits and lands, or vice versa, and by the absence of transitions. These transitions and absences of transitions represent the digital information stored on the compact disc and become the ones and zeroes of a digital representation of the recorded music. A modulation coding and formatting arrangement is used for organizing this digital data for compact storage and for error-reduced recapture.
The method used for converting the information stored on the compact disc is to rotate the disc on a spindle while the beam of a laser strikes the discrete bits of information on the compact disc and is reflected from the compact disc to a photosensor which reads the reflected signal and converts the light signal to a digital electronic signal. The electronic signal is conditioned and converted to an analog signal which is appropriately amplified and converted to sound energy through a loudspeaker. As the laser beam is directed at the disc, the reflected beam will experience changes in the reflective light pattern during the transitions from pits to lands. These changes are decoded as ones, and the absence of change is decoded as zeroes, the number of which depends on the length of the pit or land.
It was originally believed that the data stream recovered in a compact disc player was exceedingly robust, and that as a result the sound quality was unaffected by microphony, transmitted vibration, and other influences. However, designers have since shown that a number of mechanical areas can, and do, affect player sound quality, and that overall mass can also exert an influence. Other factors include the mass and decoupling of the compact disc transport itself, the dampening and mass of the disc clamps and of the support hub, plus the rigidity and the dampening of the case, for example, the top cover.
Less vibration reaching the transport means less work for the laser head in its task of maintaining focus on the fast moving data track. In turn, better tracking means that fewer power drain variations are imposed on the overall system, with the probability of lower data error rates. In fact, via a circuitous route, even corrected errors can be shown to affect sound quality through power supply interactions.
Thus, even though the compact disc has a very high theoretical capacity for high fidelity sound reproduction when sufficiently high effective sampling rates are employed, the musical quality of compact disc players has, nevertheless, been found to be limited. To some listeners, the resultant sound quality is described as harsh, grating, lacking in musical naturalness, piercing, booming, as having a notable loss of sound stage and presence and a general lack of naturalness.
A wide variety of improvements have been proposed to cure these deficiencies, all of which generally are directed at some specific problem. A list of improvement features includes special sampling techniques, improved analog-to-digital converters, improvement in filters, special modulation codes, particular formatting, phase shift compensation, anti-aliasing filters, separation of laser and spindle mechanisms from signal electronics into differently housed components, improved audio cables of special construction, special turntable supports such as isolating platforms for minimizing the effects of vibration, and many more. The vibrations caused by audio feedback and stray light have been implicated in some tests regarding audio quality degradation. Proposed vibration cures include improved sub-chassis construction, special suspensions, and special compact disc player support tables which claim to help isolate vibration-caused defects. In general, compact disc players have been said to possess immunity to such effects, but the proposed improvements all seem to make a suitable improvement in sound quality. Indeed, another cause of audio quality degradation may involve an intermodulation interaction between the vibration-induced fluctuations in the drive current requirements for the tracking servos which are working to remove such fluctuations as reflected into the digital circuitry and analog electronics through common connections such as power supplies.
There is, therefore, a need for an improved compact disc playing system which will contribute to achieving a more natural, listenable, and satisfying compact disc reproduction, especially of musical material.
One solution is proposed in U.S. Pat. No. 4,726,007, issued to McCormack. This patent proposes the use of a rigid, reinforced damper disc made of material having significant loss to vibration and low sound propagation velocity. The damper disc is arranged to be laid over the compact disc and placed in loose physical contact with its upper surface to suppress and dissipate vibrational energy from the compact disc. The damper disc is light-opaque and light-absorptive.
The quality of the sound produced by a compact disc player also requires that the data track on the compact disc be read at constant linear speed. It has become an audio standard to record and read compact discs along a spiral track at a constant linear speed by rotating the disc at variable speed. Because the track is arranged in a spiral configuration on the disc, the actual rotational speed of the compact disc is varied during the playback. Compact disc players thus operate at constant linear speed and variable angular speed. The compact disc player adjusts the angular or rotational speed of the disc during play so that the track from which information is extracted is passing through the laser beam at constant speed. In practice, the laser signal starts at the center of the compact disc and works its way out toward the outer edge. The imposition of constant linear speed results in the compact disc rotating faster at the start of play or at the center of the disc than at the end of play or at the outside of the disc.
The compact disc player attempts to accomplish a constant linear speed by altering the rotational speed of the compact disc by means of a feedback servo system which uses the laser signal as part of the speed control system. Elaborate speed control systems have been developed to attempt to assure that a constant linear speed is maintained. Examples of such servo systems are shown in U.S. Pat. No. 4,338,683, issued to Furukawa et al., U.S. Pat. No. 4,539,667, issued to Fujiie, and U.S. Pat. No. 4,611,319, issued to Naito. Each of these systems senses a synchronization signal from the compact disc playback and uses the signal through a servo mechanism to control the speed of the spindle motor driver.
While such systems are generally effective, they require that the compact disc rotate at exactly the same speed as the spindle. Any variation in speed between the compact disc and the spindle, such as slippage, causes the servo mechanism to change the speed of the spindle, which, in turn, can affect the slippage.
Compact discs are made of rigid, smooth, transparent plastic. During playback, the compact disc rests on a spindle which rotates during playback. The spindles on which compact discs rest are generally polished or finely machined hard materials, such as metal or plastic. The combination of the plastic disc and the smooth, polished spindle results in a possibility of the disc slipping with respect to the spindle during playback. Such slippage is particularly likely when the speed of the spindle changes.
The macrolinear speed of the compact disc is closely controlled in the order of a few hundred revolutions per minute. In achieving a constant linear speed, angular velocity is changed during playback by the servo speed control system. There may be dislocations associated with the servo speed control. The disc changes speed during rotation play from the faster angular to the slower angular velocity in order to have a constant linear speed. This requires frequent micro changes or slowings of the angular or actual rotational speed expressed in revolutions per minute.
This problem does not occur with conventional phonographic recordings. Conventional phonographic record players have constant angular speed. In a conventional phonograph player, once a single record is rotating at the chosen speed, there is not a practical tendency for the record to slip on its turntable since the two are continuously moving at the same angular speed or revolutions per minute.
In contrast, compact discs' angular inertia will tend to be greater than that of the spindle or drive mechanism. Micro slowings determined by the servo system of control to achieve constant linear velocity can result in a braking of the drive, which braking the inertia of the compact disc will try to overcome and result in slippage of the compact disc during play, and this slippage can generate vibration of the compact disc.
There is a need for enhancing the playback characteristics or properties of compact discs by providing means to give good adhesion of the compact disc to the spindle on which it rests when it is rotated during playback. Not only is a frictional engagement enhancement needed, but there is also a need for an energy absorption means for compact discs which will allow the vibrations energy associated with slippage (or braking as noted) to be absorbed or dampened and not transmitted to the compact disc.